Licorice root is one of the most ancient medical plants being used in the traditional Chinese, Tibetan, Indian and Arabian medicine. The most important, and well-known bioactive component of licorice root is glycyrrhizin (GL), a natural product of the class of triterpene glycosides, also called saponins. Glycyrrhetinic acid (GA) is the aglycone of GL and thus consists only of the triterpene part without the attached sugar molecules (see FIG. 1). A variety of pharmacological activities for GL and GA have been reported over the last decades comprising in vitro and in vivo studies. A good number of publications can be found in the field of steroid metabolism predominantly describing the inhibitory activity for 11β-HSDs with various pharmacological effects. GA has been widely reported as a potent inhibitor of intercellular gap-junctional communication most likely involving connexin43. Furthermore, anti-inflammatory/immunemodulatory effects were reported suggesting several targets involved in the inflammatory process. Several papers report liver protective and anti-cancer properties whereas the impact on apoptosis/oxidative stress has been discussed controversially. Finally, antibiotic and antiviral effects have been reported comprising antibacterial effects on periodontopathogenic bacteria.
The pharmacology and toxicology of GL and GA has been comprehensively reviewed [1-7].

Impact on Apoptosis and Oxidative Stress
GA is a potent inducer of mitochondrial permeability transition and can trigger the pro-apoptotic pathway. GA is a potent inhibitor of bile acid-induced apoptosis and necrosis in a manner consistent with its antioxidative effect, significantly decreases neutrophil-generated oxygen species and inhibits the generation of inflammatory mediators. Below a certain concentration, GA prevents oxidative stress and mitochondrial permeability transition but at higher concentrations GA induces oxidative stress in certain tissues. GA reveals also an effect on the protein expression of markers of oxidative stress (PAI-1 and p22phox) and scavenges oxygen free radicals in polymorphonuclear leukocytes (PMN). In addition GL stimulates DNA synthesis, proliferation in hepatocytes, and tyrosine phosphorylation of the EGF receptor and p42 MAP kinase.
Antibiotic and Antiviral Effects
GL, GA and derivatives showed inhibition of replication, growth, proliferation or specific proteins of various viral and bacterial pathogens in vitro and in vivo. Examples include SARS-coronavirus replication, influenza A virus (H1N1, H2N2, H3N2), herpes simplex virus (HSV), hepatitis C virus, hepatitis A, HIV-1-induced cytopathogenicity, hepatitis B virus (HBV), hyaluronate lyase from Streptococcus agalactiae, diverse species of periodontopathogenic and capnophilic bacteria, clarithromycin- and metronidazole-resistant strains of Heliobacter pylori, plaque formation in Japan encephalitis virus (JEV), vaccinia virus, Epstein-Barr virus (EBV), and Leishmania donovani. 
In addition GA and GL result in reduced levels of IL-10 and IL-4, but increased levels of IL-12, IFN-gamma, TNF-alpha, and inducible NO synthase.
Liver Protective Effects
GA and GL treatment significantly reduces the increase of serum transaminases induced by D-galactosamine (GalN), CCl4, or retrorsine. GA inhibits the proliferation and collagen production of hepatic stellate cells (HSGs), down-regulates the mRNA expression of type III and I procollagen, and reduces the deposition of type III and I collagen in fibrotic liver. GA also prevents the depletion of glutathione in the livers of CCl4-intoxicated mice and protects gel entrapped hepatocytes from tacrine toxicity.
GA treatment attenuates bile duct and hepatocyte damages in acute vanishing bile duct syndrome (AVBDS) rat model induced by α-naphthylisothiocyanate (ANIT).
Anti-Inflammatory and Immunmodulatory Effects
GA and GL inhibit secretory type IIA phospholipase A2 purified from the synovial fluids of patients with rheumatoid arthritis. GA inhibits the classical complement pathway at the level of C2, complement C3 is a GL-binding protein and GA induces conformational changes in C3. In the presence of GA, two trypsin-resistant fragments of C3α were immuno-precipitated with anti-C3α which could be selectively purified from the synovial fluids of patients with rheumatoid arthritis. In addition, phosphorylation of C3α by CK-2 was completely inhibited by 30 μM GA. GL (100 μM) induces conformational changes in high mobility group box (HMGB)1 and 2 and completely inhibits the phosphorylation of HMGB1/2 by PKC and CK-I.
GA significantly improved bleeding on probing and gingival inflammation in a clinical study evaluating the local application of a paste containing GA.
The anti-inflammatory activity of GA is similar to hydrocortisone on formalin-induced arthritis in albino rats. Repeated treatment with GA significantly inhibits paw edema of rats with adjuvant arthritis (AA) and croton oil-induced mouse-ear-edema, decreases T-lymphocyte ratio, reduces proliferation of synovial cells and pannus formation, and eliminates the destruction of articular cartilage in inflamed joints of AA rat.
GA suppresses TNFα-induced IL-8 production through blockade in the phosphorylation of MAPKs, following IκBα degradation and NFκB activation. GL enhances interleukin-2 (IL-2) secretion and IL-2 receptor (IL-2R) expression. In addition GL promotes tyrosine phosphorylation of p56 induced by anti-CD3. GL augments lipopolysaccharide (LPS)-induced IL-12 p40 mRNA expression, transcription of IL-12 mRNAs and IL-12-protein production. GL increases production of IL-10 in vitro and in mice with Con A-induced hepatitis. GL inhibits prostaglandin E2 production and release of [3H]arachidonic acid. GA lowers inflammatory capillary permeability, inhibits neutrophil emigration and prostaglandin E2 synthesis, and scavenges free radicals in a rat model of histamine, carrageenan, or ararachidonic acid-induced peritonitis. GA dose-dependently increases NO production and iNOS mRNA through activation of protein/DNA binding of NF-κB to its cognate site, enhances the production of nitric oxide from IFN-γ activated cells and tumor cell killing by macrophages activated with IFN-γ. This tumor cell killing is mainly by nitric oxide.
Anti-inflammatory activities of natural triterpenoids including GA have been reviewed recently.
Short Chain Dehydrogenase Reductases (SDR) and Corticoid Metabolism
GA is a potent non-competitive inhibitor of different hydroxysteroid dehydrogenases (HSD). GA inhibits 11β-HSD 1 and 11β-HSD 2 involved in the metabolism of corticosteroids, 3α-HSD involved in inflammatory processes, 3α/β,20β-HSD involved in the metabolism of androgens and progestins, 5β-HSD involved in the metabolism of cortisol, aldosterone and testosterone, and 3β-HSD involved in the metabolism of aldosterone and other steroids.
GL and GA can bind to mineralocorticoid and glucocorticoid receptors with low but sufficient affinity in order to explain the mineralocorticoid-like side effects. GA potentiates the action of aldosterone and facilitates the active transport of sodium in frog skin epithelium. GA stimulates an increase in steroid production in adrenal cells lacking intact cell junctions.
Especially the modification of corticosteroid levels by inhibition of 11β-HSD 1 and 2 by GA has been connected to numerous biological states and diseases. Examples include the reversible, gradual, constant and significant increase in systolic blood pressure, reduction in diuresis and increase in renal sodium retention, the reduction of thigh circumference and thickness of the subcutaneous fat layer in human volunteers after topical application, the reduction of metabolic detoxification of the cigarette smoke carcinogen nitrosamine 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK), the involution of the thymus and thymocyte apoptosis, the potentiation of corticosteroid effects in cultured primary human bronchial epithelial cells (PBECs), ear swelling in dinitrofluorobenzene challenged mice, human volunteer skin vasoconstrictor assay and lung tissue, the retardation of the development of autoimmune disease, as well as the increased glucose use in subregions of the hypothalamus, hippocampus, neocortex and subthalamus.
11β-HSD mRNA is expressed in neurones of the hypothalamic paraventricular nucleus (PVN) where corticotrophin-releasing factor-41 (CRF-41) is synthesized and GA decreases the release of CRF-41 into hypophysial portal blood in rats, suggesting that 11β-HSD regulates the effective corticosterone feedback signal to CRF-41 neurons.
Anticancer Effects
GA inhibits oxidative stress DMBA/TPA-induced skin tumor formation, inhibits ear edema and ornithine decarboxylase activity induced by croton oil in mice, protects against rapid DNA damage and decreases unscheduled DNA synthesis induced by benzo[a]pyrene, increases the antiproliferative effect of glucocorticoids In MCF-7 and ZR-75-1 breast cancer cells, reduces the tumor weight in rats transplanted with ‘Oberling-Guerin’ myeloma, inhibites proliferation of HepG2 human hepatoma cell line, inhibits the mutagenicity of benzo[a]pyrene, 2-aminofluorene and aflatoxin B1, and protects against tumor initiation as well as tumor promotion by 7,12-dimethylbenz[a]anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate.
GA also increases the accumulation of calcein, a fluorescent substrate of multidrug resistance protein 1 (MRP1) and of daunorubicin, a fluorescent substrate of P-glycoprotein, resulting in sensitivity to anticancer drugs, showing that GA reverses multidrug resistance.
Gap Junction Blockade and Endothelial Relaxation
GA inhibits intercellular gap-junctional communication in human fibroblasts and cultured rat neonatal cardiomyocytes, as well as type 1 or type 2A protein phosphatase-mediated Connexin43 dephosphorylation in WB-F344 rat liver epithelial cells. GA inhibites fluorescence replacement after photobleaching (FRAP) in primary chick osteocyte cultures, also indicating gap junction blockade.
GA increases the apparent cell input resistance and completely blocks membrane chloride conductance blocked while Na+ and K+ conductance are virtually unchanged.
GA in a concentration-dependent fashion attenuates EDHF-type relaxations to acetylcholine (ACh), observed in the presence of NG-nitro-L-arginine methyl ester (L-NAME) and indomethacin, modulates contractions produced by nor-adrenalin or high-K solutions and significantly reduces ACh-induced hyperpolarizations in both, endothelial and smooth muscle cells of guinea pig coronary and rat mesenteric arteries. Inhibition of the EDHF-hyperpolarization and relaxation in the smooth muscle may stem from the inhibition of endothelial cell hyperpolarization. GA quickly blocked electrical communication between smooth muscle and endothelial cells in guinea-pig mesenteric arterioles.
GA inhibits pressure-induced myogenic tone of rat middle cerebral arteries and vasopressin-induced vasoconstriction, increases input resistance in rat isolated mesenteric small arteries, desynchronised isolated smooth muscle cells, and had nonjunctional effects on membrane currents. GA significantly increases the frequency of phrenic bursts decreases the peak amplitude of integrated phrenic nerve discharge in an arterially perfused rat preparation.
GA inhibits the spike component of the action potential (AP), reduces contraction evoked by electrical stimulation, inhibits slow depolarization with superimposed APs and phasic contractions of the ureter induced by neurokinin A, and inhibits the KCl-evoked APs and phasic contractions without affecting the sustained responses in the guinea pig ureter.
GA inhibits frequencies of paced contractions, likely owing to inhibition of I-type Ca2+ channels, reduces the amplitudes of spontaneous and nerve-induced contractions, decreases phasic contractions and depolarizes resting membrane potential in murine small intestinal muscles. GA also inhibits the spread of Lucifer yellow, increases input resistance, decreases cell capacitance in interstitial cells of Cajal networks and decreased L-type Ca2+ current without affecting the voltage dependence of this current.
GA decreased the postsynaptic light response in murine retinal ganglion cells to 30% of control.
Other Effects
GA reduces the bon resorption in rheumatoid arthritis and periodontits.
GA reduces coughing in guinea-pigs by 50% compared to saline.
GA increases cytoplasmic free Ca2+ and inhibits Ca2+ increases induced by antigen, ATP, phenyephrine and thrombin. GA inhibites dexamethasone-induced increases in the histamine synthesis and histamine release. GA inhibits histidine decarboxylase and maturation of mast cells, lowers expression of PKC delta mRNA suggesting that the inhibition of histamine synthesis by GA is regulated by nPKC delta. GA significantly inhibits the degranulation of RBL-2H3 cells induced by IgE with the antigen (DNP-HSA) and rat peritoneal mast cells induced by compound 48/80. GA inhibits the passive cutaneous anaphylactic reaction as well as the scratching behaviour in mice induced by compound 48/80 and the production of IgE in ovalbumin-induced asthma mice.
GA sodium salt strongly counteracts arrhythmia induced by chloroform, lengthens the appearance time of arrhythmia induced by CaCl2, slightly retards the heart rate of rats and rabbits, and partly antagonizes the acceleration effect of isoproterenol on rabbit hearts.
GA competitively inhibits the Na+/K+-ATPase of canine kidney basolateral membranes.
GA significantly increases insulin-stimulated glucose uptake in 3T3-L1 adipocytes, glucose-stimulated insulin secretion in islets isolated from mice and induces mRNA levels of insulin receptor substrate-2, pancreas duodenum homeobox-1 and glucokinase in islets.